1. Field of the Invention
The present invention relates to a metal ion plasma generator that vaporizes a metal cathode by means of a vacuum-arc discharge to produce a metal ion plasma.
2. Description of the Background Art
An apparatus shown in FIG. 3 is known which produces a metal ion plasma by the vacuum-arc discharge, in which the apparatus is composed of a bar-form metal cathode 52 and a plate-form anode 53 in a vacuum chamber 51 (refer to a disclosure in JP-A-63-276858). The cathode 52 has an axis perpendicular to the anode 53 and is supported by a cathode holder 54. The anode 53 has an opening 55 which is coaxial with and faces with a space to the cathode 52.
The front end part projected from the cathode holder 54 is sheathed by an insulator ring 56 made of an insulating material. And, the insulating ring 56 is further sheathed by a trigger ring 57 made of a conductive material, and a trigger electrode 58 is guided inside the vacuum chamber 51 so as to touch the outer surface of the trigger ring 57.
In the foregoing construction, applying a high pulse voltage having a pulse length of some .mu. sec between the trigger ring 57 and the cathode 52 through the trigger electrode 58 will generate a trigger discharge therebetween. And then, simultaneously applying a pulse voltage having a predetermined pulse length (hereinafter, arc pulse) between the anode 53 and the cathode 52 will generate a vacuum-arc discharge between the anode 53 and the cathode 52 in which the foregoing trigger discharge is served as a starter. A point called arc spot where energy is locally concentrated emerges at the front end part of the cathode 52 through this vacuum-arc discharge. This part vaporizes and ionizes to produce a metal ion plasma. This metal ion plasma is supplied through the opening 55 of the anode 53 to a processing room (not illustrated) connecting with the foregoing vacuum chamber 51.
When the foregoing apparatus is used for forming a thin film, the foregoing metal ion plasma is supplied on a substrate disposed in the processing room. And, when the foregoing apparatus is used for a metal ion source, the foregoing metal ion plasma is supplied to an ion pullout electrode provided inside the processing room.
While a vacuum-arc discharge is generated between the cathode 52 and the anode 53, vaporized substances from the cathode gradually deposit on the surface of the insulating ring 56. As this coating phenomenon progresses, the resistance between the trigger ring 57 and the cathode 52 decreases gradually because the substances vaporized from the cathode are conductive, which finally short-circuits the trigger ring 57 and the cathode 52 to stop the arc discharge.
This coating phenomenon on the insulating ring 56 is extremely sensitive to a length of the arc pulse applied between the anode 53 and the cathode 52. In most of the cathode materials, setting the pulse length of the arc pulse to 1 msec or longer will stop the arc discharge within 2-3 minutes due to the short circuit between the trigger ring 57 and the cathode 52. Therefore, in the operation of a conventional apparatus, the pulse length of the arc pulse is set to 1 msec or shorter.
However, in the operation with the arc pulse length of 1 msec or shorter as described above, the arc spot is limited to move on the surface of the cathode 52 during the vacuum-arc discharge, thereby the cathode consumes unevenly. Such an uneven consumption of the cathode 52 causes a shorter life of the cathode and an uneven density distribution of plasma reaching at the ion pullout electrode or the like.
In order to prevent this from occurring, it is necessary to set the arc pulse length to 1 msec or longer; however in this case, it stops the arc discharge within few minutes as described above, which disables continuous operation and lowers productivity.